Cathodic protection of metal structures is well known. Substantially the metal structure is made the cathode in a circuit including a direct current source, an anode and an electrolyte between the anode and the cathode. The exposed surface of the anode is made of a material which is resistant to corrosion, for example platinum or mixed metal oxides, on a base structure made of a valve metal such as titanium or an organic polymer containing a dispersion of carbon black or graphite. There are many types of metal structures which need protection from corrosion, including steel reinforcing members in concrete, which are often referred to as "rebars". Concrete is sufficiently porous to allow passage of oxygen and liquid through it. Consequently, salt solutions, which remain in the concrete or which permeate the concrete from the outside, will cause corrosion of the rebars in the concrete. This is especially true when the electrolyte contains chloride ions, as for example in structures which are contacted by the sea water, and also in bridges, parking garages, etc. which are exposed to water containing salt used for deicing purposes or, finally, when calcium chloride has been added to the mortar as a hydration accelerator. The corrosion products of the rebars occupy a much larger volume than the metal consumed by the corrosion. As a result, the corrosion process not only weakens the rebars, but also, and more importantly, causes cracks and spalls in the concrete. It is only within the last ten or fifteen years that it has been appreciated that corrosion of rebars in concrete poses problems of the most serious kind, in terms not only of cost but also of safety. There are already many reinforced concrete structures which are unsafe or unusable because of deterioration of the concrete as a result of corrosion of the rebars, and unless some practical countermeasures to the problem are applied the number of such structures will increase dramatically over the next decade. Consequently, much efforts and expenses have been devoted to the development of methods for cathodic protection of rebars in concrete. As a result, cathodic protection has been recently proposed for the prevention against corrosion at the stage of the construction of concrete structures which are expected to be contaminated by chlorides during their lifetime ( for example bridges in mountain areas, docks, structures operating in sea environments). Cathodic protection, applied to already built new structures, comprises several steps which are time and labor consuming. In fact, it comprises making slots in the concrete to expose the rebars, installing connection cables, sandblasting the concrete surface, positioning the anodes and covering the same by a cementious overlay. If installation is carried out during the construction phase before pouring of the concrete, there would be no need for these preparation with obvious remarkable savings. The anode for cathodic protection of new structures, which should be installed on the reinforcing steel cage before concrete pouring, needs to be kept apart with appropriate insulating means and should also exhibit outstanding mechanical characteristics to avoid possible ruptures during pouring of the cement or sagging due to the weight of the concrete. In this event the anode would come into contact with the metal of the reinforcing bars causing shortcircuiting of the system. The structures of the prior art anodes are not suitable for installation as above illustrated. For example, British patent no. 2,175,609 describes an extended area anode comprising a plurality of wires in the form of an open mesh provided with an anodically active coating which may be used for the cathodic protection of steel rebars in reinforced concrete structures.
U.S. Pat. No. 4,708,888 describes a cathodic protection system using anodes having a highly expanded structure with more than 90% of void areas with respect to the empty areas.
The anode systems described in the cited patents cannot be utilized during construction before pouring of the concrete because the flimsiness of the highly expanded titanium meshes would easily result in mechanical damage and possible shortcircuit with the rebar cage during the pouring operation and subsequent vibration of the concrete.